1. Field of the Invention
The invention relates to an electronic supply device. More particularly, it relates to a device with a power factor correction circuit.
2. Description of the Prior Art
There is a known circuit for the correction of power factor without line inductance. The circuit is connected to the output of a bridge rectifier and uses rectifier diodes for the series charging and parallel discharging of two filtering capacitors. An electrical diagram of such a circuit is shown in FIG. 1. A bridge rectifier 1 receives a periodically varying voltage V.sub.AC at a pair of input terminals. A power factor correction circuit 3 is connected between the rectifier 1 and a load 2 at a pair of output terminals of the rectifier. This circuit 3 uses two filtering capacitors, C1 and C2, having the same capacitance.
A first rectifier diode D1 is connected directly between the two capacitors C1 and C2. The assembly is connected between two output terminals of the bridge rectifier.
A second rectifier diode D2 is reverse-connected in parallel with the combination of the first capacitor C1 in series with the first diode D1.
A third rectifier diode D3 is reverse-connected in parallel with the combination of the second capacitor in series with the first diode D1.
The working of such a circuit shall now be explained with reference to the curves shown in FIG. 2.
In a steady operating state, at a start of a half-wave of the line voltage V.sub.in, the diode D1 is off. The diodes D2 and D3 are on. This corresponds to the end of the period of the discharging of the capacitors C1 and C2. When the line voltage V.sub.in exceeds the charging voltage of the capacitors, the diodes D2 and D3 go to the off state. Then, when the line voltage V.sub.in exceeds the sum of the charging voltages of the two capacitors (Vc1+Vc2), the diode D1 becomes conductive (T1) and the two capacitors are charged in series until the line voltage reaches its peak value Vc (T2). The diode D1 then goes back to the off state. The two capacitors are each charged at Vc/2 (being identical capacitors).
The line voltage, which then decreases, becomes lower than this charging voltage Vc/2: the diodes D2 and D3 therefore come on, while D1 remains off (T3). The capacitors are again parallel-connected. The capacitor C1 supplies the load through the diode D3 and the capacitor C2 supplies the load through the diode D2. This process stops as soon as the line voltage V.sub.in again starts increasing (at the next half-wave) and becomes greater than the voltage of each capacitor: the diodes D2 and D3 go back to the off state, the diode D1 remains off. The system is then at T0, and the cycle then repeats. The current waveform I.sub.in shown in FIG. 2 is obtained.
Between T0 and T1, it is the mains supply system (V.sub.AC) that directly supplies the load (with D1, D2 and D3 off). The shape of the current waveform for a value of power P.sub.out consumed in the load 2 is given by the relationship: EQU I.sub.in(t) =P.sub.out /V.sub.in(t)
For Pout constant, between T0 and T1, V.sub.in increases and I.sub.in decreases.
Between T1 and T2, the capacitors are charged. On top of the current consumed in the load 2 (shown in dashes), there is superimposed the charging current for the capacitors.
Between T2 and T3, the charging of the capacitors, each at half of the peak voltage Vc, is over. The current I.sub.in is only the current consumed in the load 2 and the waveform of the current is given by the relationship: EQU I.sub.in(t) =P.sub.out /V.sub.in(t).
The line voltage decreases and I.sub.in decreases (with P.sub.out constant).
Finally, between T3 and T0, it is the capacitors C1 and C2 that supply the load 2. The current I.sub.in drawn from the rectifier is zero.
The circuit 3 therefore makes it possible to increase the angle of flow of the bridge rectifier. The waveform of the current I.sub.in is spread over the voltage half-wave with three phases of conduction: [T0-T1], [T1-T2] and [T2-T3]. In this way, the power factor of the device (namely the ratio of the actual power to the apparent total power) is improved since the line is forced to consume current during the most significant part of the voltage wave, namely when the instantaneous value of the line voltage exceeds half of the peak value Vc.
However, for the charging of the capacitors, there is a drawing of charging current which gives rise to a steep leading edge of the line current. There is therefore a current peak. This corresponds to non-negligible low frequency harmonic contents that limit the value of the power factor (with a supply of power at harmonic frequencies different from the line frequency).